This application claims the benefit of Korean Application No. 47183/1999, filed in the Republic of Korea on Aug. 16, 2000, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device, and more particularly, to a method for fabricating a gate oxide film of a semiconductor device by which semiconductor devices having different electrical characteristics can be implemented in the same chip.
2. Description of the Background Art
Recently, as the degree of integration of a semiconductor, in particular, a DRAM (dynamic random access memory) increases, it is often the case that a transistor of a memory cell unit and a transistor of a peripheral circuit have a different operating voltage with each other. In other words, the transistor of the memory cell unit fabricated with a fine line width operates at a voltage less than 1.8V, and the transistor of the peripheral circuit operates at a voltage of 3.3V or 5V for matching with exterior system equipment.
Accordingly, as devices having different operating voltages are formed in the same chip, there occurs a problem that a gate oxide film of the transistors formed in the same semiconductor chip must have different thicknesses.
Methods conventionally known as a method for forming a gate electrode having different thicknesses in the same chip will now be described.
First, FIGS. 1A through 1D illustrate a method for fabricating a gate oxide film by a dual step oxidation process.
As illustrated in FIG. 1A, a semiconductor substrate 10 is prepared.
Next, as illustrated in FIG. 1B, a first gate oxide film 11 is formed on the top surface of the semiconductor substrate 10.
Next, as illustrated in FIG. 1C, the first gate oxide film 11 of a portion on which a relatively thin gate oxide film is to be selectively etched and removed to thereby expose parts of the top surface of the semiconductor substrate 10.
Next, as illustrated in FIG. 1D, a second gate oxide film 12 is formed on the top surface of the first gate oxide film 11 and the top surface of the semiconductor device 10.
Besides the above-said method using the dual step oxidation process, there is a method for fabricating a gate oxide film using an ion implantation process. This method will now be described with reference to FIGS. 2A through 2D and FIGS. 3A through 3D.
First, FIGS. 2A through 2D illustrates a method for fabricating a gate oxide film using a nitrogen ion implantation process.
As illustrated in FIG. 2A, a semiconductor substrate 20 is prepared.
Next, as illustrated in FIG. 2B, a screen oxide film 21 is formed on the top surface of the semiconductor substrate 20. Then, an ion implantation mask 22 is formed on the screen oxide film 21 of a portion on which a relatively thick oxide film is to be formed. Then, nitrogen (N2) ions are implanted into the semiconductor substrate 20 of a portion being not covered with the ion implantation mask 22.
Next, as illustrated in FIG. 2C, the screen oxide film 21 and the ion implantation mask are removed.
Next, when a gate oxide film 23 is formed on the top surface of the semiconductor substrate 20, as illustrated in FIG. 2D, a thin oxide film 23a is formed on a portion into which nitrogen ions are implanted, because oxidation is restrained, and a relatively thick oxide film 23b is formed on a portion into which nitrogen ions are not implanted.
In addition, a method for fabricating a gate oxide film using a fluoride ion implantation process will now be described with reference to FIGS. 3A through 3D.
First, as illustrated in FIG. 3A, a semiconductor substrate 30 is prepared.
Next, as illustrated in FIG. 3B, a screen oxide film 31 is formed on the top surface of the semiconductor substrate 30. Next, an ion implantation mask 32 is formed on the top surface of the screen oxide film 31 of a portion on which a relatively thin gate oxide film is to be formed. Then, fluoride ions are implanted into the semiconductor substrate 30 using the ion implantation mask 32.
Next, as illustrated in FIG. 3C, the ion implantation mask 32 and the screen oxide film 31 are removed.
Next as illustrated in FIG. 3D, a gate oxide film 33 having different thicknesses is formed on the top surface of the semiconductor substrate 30 by oxidation of the semiconductor substrate 30. That is, a thick gate oxide film is formed on the top surface of the semiconductor of a portion into which fluoride ions are implanted, and a thin gate oxide film is formed on a portion into which fluoride ions are not implanted.
However, the above-described conventional methods for fabricating a gate oxide film has the following problems. First, the method for fabricating a gate oxide film by the dual step oxidation process has a complicated procedure, and a peripheral portion of a fabricated, thick gate oxide film becomes thinner and a breakdown is easily occurred on a thinned portion.
Second, in case of the fluoride ion implantation process, since a large amount of fluoride ions must be implanted in order to make the thickness of a gate oxide film different according to its portion, the semiconductor substrate is largely damaged to thus increase the amount of leakage current.
Third, the nitrogen implantation process is disadvantageous in that, in case of forming a gate oxide film on the top surface of the semiconductor substrate into which nitrogen ions are implanted, the gate oxide film is degraded, although it is advantageous in that a smaller amount of ions can be implanted as to compared to the fluoride ion implantation process, which rather decreases the amount of leakage current.
Accordingly, the present invention provides a method for fabricating a gate oxide film of a semiconductor device having a small leakage current amount and a high reliability by reducing the concentration of nitrogen in the gate oxide film when a gate oxide film having different thicknesses according to its portion is fabricated on the top surface of a semiconductor substrate using a nitrogen ion implantation process.
A method for fabricating a gate oxide film of a semiconductor device according to the present invention includes the steps of: forming a screen oxide film on the top surface of a semiconductor substrate; forming an ion implantation mask on parts of the top surface of the screen oxide film; implanting nitrogen ions into the semiconductor substrate using the ion implantation mask; removing the ion implantation mask and the screen oxide film; forming an oxide film on the top surface of the semiconductor substrate; and annealing the semiconductor substrate.
There is provided a method for fabricating a gate oxide film of a semiconductor device according to the present invention which further includes a pre-annealing step after the ion implantation step.
A method for fabricating a gate oxide film of a semiconductor device according to the present invention includes the pre-annealing step by a process of annealing at 500-900xc2x0 C. by a furnace annealing method.
In a further aspect of the invention, there is provided a method for fabricating a gate oxide film of a semiconductor device wherein the pre-annealing step is a process of annealing at 850-1200xc2x0 C. by a rapid thermal annealing method.
In another aspect of the invention, there is provided a method for fabricating a gate oxide film of a semiconductor device wherein the oxide film formation step being a method for thermal oxidation at a furnace of 700-950xc2x0 C.
A method for fabricating a gate oxide film of a semiconductor device according to the present invention includes the oxide film formation step being a method for thermal oxidation at 850-1200xc2x0 C. at the rapid thermal annealing method.
A method for fabricating a gate oxide film of a semiconductor device according to the present invention includes the annealing step being performed by means of the rapid thermal annealing method in a N2O atmosphere at a temperature of 900-1200xc2x0 C. for about less than five minutes.
The annealing step can also be performed by means of the thermal annealing method in an O3 atmosphere at a temperature of 400-1200xc2x0 C. for about five minutes.
The annealing step can also be performed by means of the furnace annealing method in a N2O atmosphere at a temperature of 850-1200xc2x0 C. for about one hour.
In another aspect of the invention, there is provided a method for fabricating a gate oxide film of a semiconductor device according to the present invention wherein the annealing step is performed by means of the rapid thermal annealing method in a N2O atmosphere at a temperature of 900-1200xc2x0 C. for about less than five minutes.
Additional advantages, aspects and features of the invention will become more apparent from the description which follows.